Since May, hackers have been penetrating the computer networks of companies that operate nuclear power stations and other energy facilities, as well as manufacturing plants in the United States and other countries.

Among the companies targeted was the Wolf Creek Nuclear Operating Corporation, which runs a nuclear power plant near Burlington, Kan., according to security consultants and an urgent joint report issued by the Department of Homeland Security and the Federal Bureau of Investigation last week.

The joint report was obtained by The New York Times and confirmed by security specialists who have been responding to the attacks. It carried an urgent amber warning, the second-highest rating for the sensitivity of the threat.

The report did not indicate whether the cyberattacks were an attempt at espionage — such as stealing industrial secrets — or part of a plan to cause destruction. There is no indication that hackers were able to jump from their victims’ computers into the control systems of the facilities, nor is it clear how many facilities were breached.

Wolf Creek officials said that while they could not comment on cyberattacks or security issues, no “operations systems” had been affected and that their corporate network and the internet were separate from the network that runs the plant.

In a joint statement with the F.B.I., a spokesman for the Department of Homeland Security said, “There is no indication of a threat to public safety, as any potential impact appears to be limited to administrative and business networks.”

The hackers appeared determined to map out computer networks for future attacks, the report concluded. But investigators have not been able to analyze the malicious “payload” of the hackers’ code, which would offer more detail into what they were after.

John Keeley, a spokesman for the Nuclear Energy Institute, which works with all 99 electric utilities that operate nuclear plants in the United States, said nuclear facilities are required to report cyberattacks that relate to their “safety, security and operations.” None have reported that the security of their operations was affected by the latest attacks, Mr. Keeley said. Full article.